This Couldn't Happen! III

The Red Sox and Yankees were playing a game at Yankee Stadium. It was all tied up in the top of the ninth inning 5-5. David Ortiz stepped up to the plate and hit a walk-off home run. The Red Sox won the game!

Why couldn't this happen?

Hint

A walk-off home run is a home run that ends the game because there is no way for the other team to comeback.Hide